Astronomers discover less “metallic” star system in Milky Way

A structure identified in the Milky Way has a lower proportion of heavy elements than any other known star system in our galaxy. The discovery, described in a scientific article published this Wednesday (5) in the journal nature, was carried out through the Gemini Observatory, a program of the United States National Research Laboratory in Optical-Infrared Astronomy (NOIRLab).

According to observations, the stars in this flux were torn from an ancient star cluster and are relics from the early days of the Milky Way, which might provide data on the formation of the first stars.

An international team of researchers, including members of the European Union, Canada and Russia, is tasked with discovering the C-19, as the star flux is called, which lies south of the Milky Way. Its orbit extends regarding 20,000 light years from the galactic center to its closest position and regarding 90,000 light years from its furthest point.

This stellar flow, known as C-19, spans a wide area of ​​the night sky, regarding 30 times the width of a full moon.

The star system occupies an area equivalent to 30 full moons in the Milky Way

Scientists say the star system spans an impressive area of ​​the night sky – regarding 30 times the width of a full moon – although it is not visible to the naked eye.

Using the Gemini North telescope, located in Hawaii, and the Gemini Remote Access to CFHT SPADES Spectrograph (GRACES) instrument, both from the Gemini Observatory, the team realized that C-19 is a remnant of ‘a globular cluster.

Red globular clusters were previously thought to have no less than 0.2% metal, but C-19 has an unprecedented level: less than 0.05% metallicity.

The discovery that a weak metallic flux comes from a globular cluster has implications for the formation of stars, star clusters and galaxies in the early universe.

Animation shows how a globular star cluster, orbiting a galaxy still forming the Milky Way, may have been torn by gravity from the developing galaxy to become the C-19 stellar flux.

In fact, the very existence of this flux indicates that the globular clusters and the first building blocks of the Milky Way must have been able to form in environments poor in metals, before successive generations of stars fueled the universe. in heavier elements.

“We didn’t know if there were globular clusters with so few heavy elements. Some theories even suggested that they might not be formed, ”commented Nicolas Martin, from the Strasbourg Astronomical Observatory, the study’s main author. “Other theories suggest that they all died out a long time ago, making this a foundational discovery for our understanding of star formation in the early universe. “

Team members initially detected C-19 in data from the Gaia satellite using an algorithm they designed specifically to detect stellar flux. The stars of C-19 have also been identified by the Pristine Survey – a search for the least metallic stars in and around the Milky Way using the Canada-France-Hawaii Telescope – as interesting enough to warrant follow-up observations. .

Distribution of very dense star groups in the Milky Way, called globular clusters, superimposed on a map of the galaxy established from data obtained from the Gaia satellite. Each point represents a cluster of a few thousand to several million stars. The color of the dots shows their metallicity, that is to say their abundance of heavy elements compared to the Sun. C-19 stars are indicated by light blue symbols. Credit: N. Martin / Strasbourg Astronomical Observatory / CNRS; Canada-France-Hawaii / Coelum Telescope; ESA / Gaïa / DPAC

To identify the origin of the constituent stars of C-19, astronomers needed the detailed spectra of GRACES. The team also collected data using a spectrograph mounted at the Gran Telescopio Canarias in La Palma, Canary Islands.

“GRACES has provided the critical clues that C-19 is an interrupted globular cluster and not the most common interrupted dwarf galaxy,” said Kim Venn of the University of Victoria, principal investigator of GRACES observations. “We already knew that this was a very poor metal flux, but identifying it as a globular cluster required the precision of the metallicity and the detailed chemical abundances available only with high-resolution spectra. “

The new study also suggests that C-19 must have formed from the very first generations of stars, making the cluster a remarkable relic of the days when the first star clusters were formed.

Therefore, this discovery improves our understanding of the formation of stars and star clusters that appeared shortly following the Big Bang, and provides a natural laboratory for studying the oldest structures in galaxies.

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